Previous work on leukocytes performed in this laboratory utilizing electron microscopy and cytochemistry has provided new information on the orderly step-wise assembly of cytoplasmic organelles related to leukocyte differentiation and function. We shall continue to investigate the qualitative changes which occur during the differentiation of granulocytes, monocytes (macrophages), and megakaryocytes by analyzing the fine structure, time of appearance, mode of formation, chemical contents, fate, and function of their cytoplasmic granules. Major techniques to be applied are light microscopy of intact biopsy specimens, electron microscopy of enzyme-and immunohistochemical preparations, and tissue culture. Specifically, the present objectives are: 1) to further characterize the neutrophil and monocyte storage granule contents by enzyme- and immunohistochemistry to determine if subpopulations exist within the two basic types; 2) to further characterize, both in vitro and in vivo, the stages of differentiation of mononuclear phagocytes (promonocytes yields monocytes yields macrophages (resident, elicited, and activated)) by determining the time of synthesis, and sites of storage and, in some cases, exocytosis of peroxidase, lysosomal enzymes, lyxozyme, and possibly other products such as elastase, cathepsin, and plasminogen activator; 3) to identify the primary lysosome and its fate in human megakaryocytes and platelets, rat basophils and mast cells, and differentiating human erythroblasts; 4) to study "stromal" cells in intact bone marrow using cytochemical "marker" enzymes on 1 micron plastic-embedded sections examined by light microscopy, and, in addition, electron microscopy. The application of these techniques to stromal cells will allow their identification with certainty and may provide better understanding of their function in normal and damaged marrow; 5) to construct a "morphologic model" of granulopoiesis under normal and inflammatory states, to be incorporated with the kinetic models; 6) to further explore the phenomenon of phagocytosis, degranulation, and changes in pH of the phagocytic vacuole. Compositely, this work will lead to a better understanding of the functions and interrelationships of leukocytes in host defense.